Nitrile rubber composition, cross-linkable nitrile rubber composition and cross-linked rubber

ABSTRACT

Disclosed herein are a nitrile rubber composition comprising a nitrile rubber (a) including an α,β-ethylenically unsaturated nitrile monomer unit and having iodine value of 100 or less, an α,β-ethylenically unsaturated carboxylic acid metallic salt (b), and silane-treated fumed silica and/or calcinated wet silica (c); a cross-linkable nitrile rubber composition comprising the above nitrile rubber composition and a crosslinking agent (d); and a cross-linked rubber obtained by crosslinking the cross-linkable nitrile rubber composition. The present invention can provide a nitrile rubber composition and a cross-linkable nitrile rubber composition, capable of giving a highly-saturated cross-linked nitrile rubber having high tensile stress and excellent heat resistance, and a cross-linked rubber thereof.

FIELD OF THE INVENTION

The present invention relates to a nitrite rubber composition and across-linkable nitrite rubber composition capable of giving ahighly-saturated cross-linked nitrile rubber having high tensile stressand excellent heat resistance, and a cross-linked rubber thereof.

DESCRIPTION OF THE RELATED ART

As a rubber having excellent oil resistance, heat resistance and ozoneresistance, nitrite group containing highly-saturated copolymer rubber(i.e. “highly-saturated nitrite rubber” including hydrogenated nitrilerubber) has been known, and the cross-linked rubber thereof has beenused as a material for various automotive rubber products such as abelt, hose, gasket, packing and oil-seal. Recently, required qualitiesof the material rubbers used for automotive rubber products have becomefurther severe, and particularly in a bulk rubber product other than afiber impregnated body or metal composite body, further excellent heatresistance has been demanded since temperature in an engine room iseasier to increase due to downsized and souped-up automobile engine.

In response to this, Patent Article 1 proposes a cross-linked rubberobtained by blending silica and zinc methacrylate with ahighly-saturated nitrite rubber, and crosslinking it by a crosslinkingagent. However, the cross-linked rubber has defects such as insufficientimprovement in heat resistance and reduction in tensile stress.

-   Patent Article 1: The Japanese Unexamined Patent Publication    2006-84010.

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

A purpose of the present invention is to provide a nitrile rubbercomposition and a cross-linkable nitrile rubber composition capable ofgiving a highly-saturated cross-linked nitrile rubber having hightensile stress and excellent heat resistance, and a cross-linked rubberthereof.

Means for Solving the Problem

The present inventors found the following, as a result of keenexaminations, and came to complete the present invention based on thefindings:

(i) insufficient improvement in heat resistance and reduction in tensilestress shown in the cross-linked rubber of the above Patent Article 1(Japanese Unexamined Patent Publication 2006-84010) are caused by usingintact wet silica without any particular treatment (specifically, inPatent Article 1, “Nipsil VN-3 manufactured by Nippon Silica IndustrialCo Ltd.” is used.), or using hydrophobized silica obtained by treatingwet silica with organic silane (specifically, in Patent Article 1,“Nipsil SS-10 manufactured by Nippon Silica Industrial Co Ltd.” isused.); and

(ii) in contrast, by using specifically-treated silica in combinationwith a highly-saturated nitrile rubber and α,β-ethylenically unsaturatedcarboxylic acid metallic salt, the above purpose can be attained.

Namely, according to the present invention, there is provided a nitrilerubber composition comprising a nitrile rubber (a) including anα,β-ethylenically unsaturated nitrile monomer unit and having iodinevalue of 100 or less, an α,β-ethylenically unsaturated carboxylic acidmetallic salt (b), and silane-treated fumed silica and/or calcinated wetsilica (c).

Preferably, a content of said α,β-ethylenically unsaturated carboxylicacid metallic salt (b) is 3 to 120 parts by weight per 100 parts byweight of said nitrile rubber (a). Preferably, a content of saidsilane-treated fumed silica and/or calcinated wet silica (c) is 5 to 300parts by weight per 100 parts by weight of said nitrile rubber (a).

Preferably, said α,β-ethylenically unsaturated carboxylic acid metallicsalt (b) is a zinc salt and/or magnesium salt of unsaturatedmonocarboxylic acid, more preferably zinc dimethacrylate. Preferably,said silane-treated fumed silica is obtained by surface treatment offumed silica with (meth)acryloxy group containing silane and/or alkylgroup containing halogenated silane. Preferably, said calcinated wetsilica is adjusted to have a concentration of surface silanol groups of3 groups/nm² or less by calcinating wet silica.

Also, according to the present invention, there is provided across-linkable nitrite rubber composition comprising any one of theabove nitrile rubber compositions and a crosslinking agent (d).

In the cross-linkable nitrite rubber composition of the presentinvention, preferably, a content of said crosslinking agent (d) is 0.2to 20 parts by weight per 100 parts by weight of the nitrile rubber (a).Also, said crosslinking agent (d) is preferably an organic peroxide.

Further, according to the present invention, there is provided across-linked rubber obtained by crosslinking any one of the abovecross-linkable nitrite rubber compositions. The cross-linked rubber ofthe present invention is preferably a belt or seal material.

Effects of the Invention

According to the present invention, there are provided a nitrile rubbercomposition and a cross-linkable nitrite rubber composition capable ofgiving a highly-saturated cross-linked nitrile rubber having hightensile stress and excellent heat resistance, and a cross-linked rubberthereof.

BEST MODE FOR WORKING THE INVENTION

A nitrile rubber composition of the present invention comprises anitrile rubber (a) including an α,β-ethylenically unsaturated nitrilemonomer unit and having iodine value of 100 or less, anα,β-ethylenically unsaturated carboxylic acid metallic salt (b), andsilane-treated fumed silica and/or calcinated wet silica (c).

Also, a cross-linkable nitrile rubber composition of the presentinvention comprises the above nitrile rubber composition and acrosslinking agent (d).

Note that a rubber including an α,β-ethylenically unsaturated nitrilemonomer unit may be referred to as “nitrile rubber” as described above.

Nitrile Rubber Composition

Nitrile Rubber (a)

The nitrile rubber (a) used in the present invention includes anα,β-ethylenically unsaturated nitrile monomer unit and has iodine valueof 100 or less. A monomer forming the α,β-ethylenically unsaturatednitrile monomer unit of the nitrile rubber (a) may be anyα,β-ethylenically unsaturated compound having a nitrile group withoutlimitation, and there may be mentioned acrylonitrile;α-halogenoacrylonitrile such as α-chloroacrylonitrile andα-bromoacrylonitrile; α-alkylacrylonitrile such as methacrylonitrile;etc. Among these, acrylonitrile and methacrylonitrile are preferable. Asthe α,β-ethylenically unsaturated nitrile monomer, a plurality of thesemay be used in combination.

A content of the α,β-ethylenically unsaturated nitrile monomer units inthe nitrile rubber (a) is preferably 10 to 60 wt %, more preferably 15to 55 wt % and particularly preferably 20 to 50 wt % per 100 wt % of allmonomer units. When the content of the α,β-ethylenically unsaturatednitrile monomer units is too small, oil resistance of the obtainedcross-linked rubber may be reduced, and in contrast, too large contentmay cause to reduce cold resistance.

The nitrile rubber (a) normally includes, a diene monomer unit and/orα-olefin monomer unit as well as the α,β-ethylenically unsaturatednitrile monomer unit to provide rubber elasticity of the obtainedcross-linked rubber.

As a diene monomer forming the diene monomer unit, there may bementioned a conjugated diene with carbon number of 4 or more such as1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene;a nonconjugated diene with carbon number of preferably 5 to 12 such as1,4-pentadiene, 1,4-hexadiene, vinyl norbornene and dicyclopentadiene;etc. Among these, conjugated diene is preferable, and 1,3-butadiene ismore preferable.

An α-olefin monomer forming the α-olefin monomer unit preferably hascarbon number of 2 to 12, and there may be mentioned, for example,ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene,etc.

A content of the diene monomer unit and/or α-olefin monomer unit in thenitrile rubber (a) is preferably 20 to 90 wt %, more preferably 30 to 85wt %, further preferably 40 to 80 wt % and particularly preferably 50 to80 wt % per 100 wt % of all monomer units. When the content of thesemonomer units are too small, elasticity of the obtained cross-linkedrubber may be decreased; and too large content may cause to deteriorateheat resistance and chemical stability.

The nitrite rubber (a) may also includes other monomer unitcopolymerizable to the α,β-ethylenically unsaturated nitrile monomer,and diene monomer and/or α-olefin monomer. As a monomer forming othermonomer unit, there may be mentioned an aromatic vinyl monomer, afluorine containing vinyl monomer, α,β-ethylenically unsaturatedmonocarboxylic acid and ester thereof, an α,β-ethylenically unsaturatedpolyvalent carboxylic acid, an α,β-ethylenically unsaturated polyvalentcarboxylic acid monoester and polyvalent ester, an α,β-ethylenicallyunsaturated polyvalent carboxylic acid anhydride, a cross-linkablemonomer, copolymerizable anti-aging agent, etc.

As the aromatic vinyl monomer, styrene, α-methylstyrene, vinylpyridine,etc., may be mentioned.

As the fluorine containing vinyl monomer, fluoroethylvinyl ether,fluoropropylvinyl ether, o-trifluoromethylstyrene, vinylpentafluorobenzoate, difluoroethylene, tetrafluoroethylene, etc., may bementioned.

As the α,β-ethylenically unsaturated monocarboxylic acid, for example,acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, etc., maybe mentioned.

As the α,β-ethylenically unsaturated monocarboxylic acid ester, forexample, ethyl (meth)acrylate (indicating ethyl acrylate or ethylmethacrylate; the same goes for the following compounds such as butyl(meth)acrylate), butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, etc.

As the α,β-ethylenically unsaturated polyvalent carboxylic acid, forexample, maleic acid, fumaric acid, itaconic acid, etc., may bementioned.

As the α,β-ethylenically unsaturated polyvalent carboxylic acidmonoester, for example, monomethyl maleate, monoethylmaleate,monobutylmaleate, monocyclopentylmaleate, monoethyl itaconate,monomethylcyclopentyl itaconate, etc. may be mentioned.

As the α,β-ethylenically unsaturated polyvalent carboxylic acidpolyvalent ester, for example, dimethyl maleate, di-n-butyl fumarate,dimethyl itaconate, di-2-ethylhexyl itaconate, etc., may be mentioned.

As the α,β-ethylenically unsaturated polyvalent carboxylic anhydride,for example, maleic anhydride, itaconic anhydride, etc., may bementioned.

As the cross-linkable monomer, there may be mentioned a polyfunctionalethylenically unsaturated monomer including a divinyl compound such asdivinylbenzene; di(meth)acrylates such as diethylene glycoldi(meth)acrylate and ethylene glycol di(meth)acrylate;tri(meth)acrylates such as trimethylolpropane tri(meth)acrylate; etc.,and a self-cross-linkable monomer such as N-methylol (meth)acrylamideand N,N′-dimethylol (meth)acrylamide.

As the copolymerizable anti-aging agent, there may be exemplifiedN-(4-anilinophenyl)acrylamide, N-(4-anilinophenyl)methacrylamide,N-(4-anilinophenyl)cinnamamide, N-(4-anilinophenyl)crotonamide,N-phenyl-4-(3-vinylbenzyloxy)aniline,N-phenyl-4-(4-vinylbenzyloxy)aniline, etc.

These other copolymerizable monomers may be used in combination. Acontent of these other monomer units of the nitrile rubber (a) ispreferably 60 wt % or less, more preferably 50 wt % or less andparticularly preferably 10 wt % or less per 100 wt % of all monomerunits.

The nitrile rubber (a) used in the present invention has the iodinevalue of 100 or less, preferably 80 or less, more preferably 60 or lessand particularly preferably 30 or less. When the iodine value of thenitrile rubber (a) is too high, ozone resistance of the obtainedcross-linked rubber may be reduced.

Also, Mooney viscosity [ML₁₊₄ (100° C.)] of the nitrile rubber (a) usedin the present invention is preferably 15 to 200, more preferably 30 to150 and particularly preferably 45 to 120. When the Mooney viscosity ofthe nitrile rubber (a) is too low, mechanical properties of the obtainedcross-linked rubber may be reduced. In contrast, too high Mooneyviscosity may cause to reduce workability of the nitrile rubbercomposition.

A production method of the nitrile rubber (a) used in the presentinvention is not particularly limited. In general, a method tocopolymerize the α,β-ethylenically unsaturated nitrite monomer, dienemonomer and/or α-olefin monomer, and optionally-added othercopolymerizable monomer thereto is convenient and preferable. As apolymerization method, any one of known polymerization method, such asemulsion polymerization method, suspension polymerization method, bulkpolymerization method and solution polymerization method, can be used,and emulsion polymerization method is preferable because it is easy tocontrol the polymerization.

Also, when iodine value of the copolymer obtained by copolymerization ishigher than the above range, the copolymer may be hydrogenated(hydrogenation reaction). A hydrogenation method is not particularlylimited and any known method may be applicable.

α,β-Ethylenically Unsaturated Carboxylic Acid Metallic Salt (b)

The α,β-ethylenically unsaturated carboxylic acid metallic salt (b) usedin the present invention is a salt of α,β-ethylenically unsaturatedcarboxylic acid and metal.

As an α,β-ethylenically unsaturated carboxylic acid forming theα,β-ethylenically unsaturated carboxylic acid metallic salt (b), thosehaving at least univalent carboxyl group (i.e. those having at least one“—COOH group” which forms no ester bond and the like) for generating ametallic salt may be mentioned. Specific examples may includeunsaturated monocarboxylic acid, unsaturated dicarboxylic acid,unsaturated dicarboxylic acid monoester, etc.

As the unsaturated monocarboxylic acid, acrylic acid, methacrylic acid,etc., may be mentioned. As the unsaturated dicarboxylic acid, maleicacid, fumaric acid, itaconic acid, etc, may be mentioned. As theunsaturated dicarboxylic acid monoester, monomethyl maleate, monoethylmaleate, monomethyl itaconate, monoethyl itaconate, etc., may bementioned. Among these ethylenically unsaturated carboxylic acids,ethylenically unsaturated carboxylic acids having no ester group arepreferable, unsaturated monocarboxylic acids are more preferable, andmethacrylic acid is particularly preferable.

As a metal forming the α,β-ethylenically unsaturated carboxylic acidmetallic salt, any of those formable of a salt with the aboveα,β-ethylenically unsaturated carboxylic acid can be used withoutlimitation, and various metals can be used. Among these, zinc,magnesium, calcium, barium, titanium, chromium, iron, cobalt, nickel,aluminum, tin and lead are preferable, zinc, magnesium, calcium andaluminum are more preferable, and zinc and magnesium are particularlypreferable.

The α,β-ethylenically unsaturated carboxylic acid metallic salt (b) canbe obtained by reacting the α,β-ethylenically unsaturated carboxylicacid with said metallic compound to form a salt. As the metalliccompound used in this reaction, an oxide, hydroxide or carbonate may bementioned.

An amount of the metal, used for forming the above salt, is preferably0.2 to 3 moles, more preferably 0.3 to 2.5 moles and particularlypreferably 0.4 to 2 moles with respect to 1 mole of carboxyl group inthe α,β-ethylenically unsaturated carboxylic acid. When the amount ofthe α,β-ethylenically unsaturated carboxylic acid is too large (metalamount is too small), an odor of the residual monomers in the nitrilerubber composition may be strong. On the other hand, when theα,β-ethylenically unsaturated carboxylic acid is too small in amount(metal amount is too large), strength of the obtained cross-linkedrubber may be reduced.

The α,β-ethylenically unsaturated carboxylic acid metallic salt (b) maybe blended to the rubber in the form of a preliminarily-formed metallicsalt when preparing the nitrile rubber composition by kneading togetherwith other components constituting the nitrile rubber composition suchas the nitrile rubber (a). Alternatively, the metallic salt may beformed without preliminary reaction prior to the kneading process byblending said α,β-ethylenically unsaturated carboxylic acid and saidmetallic compound when kneading with other components constituting thenitrile rubber composition, and reacting them in the kneading process.

A content of the α,β-ethylenically unsaturated carboxylic acid metallicsalt (b) in the nitrile rubber composition of the present invention isnot particularly limited, but is preferably 3 to 120 parts by weight,more preferably 5 to 100 parts by weight and particularly preferably 10to 60 parts by weight per 100 parts by weight of the nitrile rubber (a).When the content of the component (b) in the nitrile rubber compositionof the present invention is too small, the obtained cross-linked rubbermay be inferior in strength properties; and in contrast, too largecontent may cause to reduce elongation.

Silane-Treated Fumed Silica and/or Calcinated Wet Silica (c)

The nitrile rubber composition of the present invention includessilane-treated fumed silica and/or calcinated wet silica (c) in additionto the above component (a) and component (b). Namely, the nitrile rubbercomposition of the present invention includes, as the component (c), anyone or both of silane-treated fumed silica and calcinated wet silica. Inthe nitrite rubber composition of the present invention, it is necessaryto add silane-treated fumed silica and/or calcinated wet silica (c)preliminarily prepared in the respective forms when forming thecomposition.

For example, in the process of preparing the nitrile rubber compositionof the present invention, if mixing silane coupling agent and fumedsilica, i.e. materials for making silane-treated fumed silica, into thenitrile rubber (a) together with other components constituting thenitrile rubber composition, the obtained cross-linked rubber may beinsufficient in improvement effect of heat resistance.

The silane-treated fumed silica is obtained by forming a dry silica(fumed silica) by a method to hydrolyze silicon tetrachloride athigh-temperature in oxyhydrogen flame (high-temperature flame generatedby belching hydrogen and oxygen simultaneously by using blowpipe, andigniting), and treating the surface thereof with a predetermined silanecompound. Note that BET specific surface area of the fumed silica ispreferably 10 to 500 m²/g, more preferably 30 to 300 m²/g andparticularly preferably 50 to 200 m²/g. By using the fumed silica havingBET specific surface area within the range, improvement effects oftensile stress and heat resistance in the obtained cross-linked rubbermay be further significant.

The silane-treated fumed silica is obtained by surface treatment offumed silica with a silane compound, and the surface treatment with thesilane compound allows to hydrophobize a hydrophilic silanol group inthe surface of the fumed silica and to improve affinity between thefumed silica and the nitrile rubber (a). A type of the silane compoundused in the surface treatment is not particularly limited, but(meth)acryloxy group containing silane and alkyl group containinghalogenated silane are preferable.

As the (meth)acryloxy group containing silane, acryloxysilane,diacryloxysilane, methacryloxysilane, dimethacryloxysilane, etc., may bementioned. As the alkyl group containing halogenated silane, methyltrichlorosilane, dimethyl dichlorosilane, trimethyl chlorosilane, ethyltrichlorosilane, diethyl dichlorosilane, triethyl chlorosilane, etc.,may be mentioned. Among these, methacryloxysilane and dimethyldichlorosilane are preferable.

The calcinated wet silica is prepared by firing wet silica. As the wetsilica used for the calcinated wet silica, the one synthesized byneutralization of sodium silicate and mineral acid (normally sulfuricacid), etc., may be mentioned. For example, any silica can be used,either obtained by a precipitation method to perform neutralization inan alkaline side, or obtained by a gel method to perform neutralizationin an acid side.

As for firing conditions of the wet silica to obtain the calcinated wetsilica, the wet silica are preferably heated at 500 to 1,000° C. for 30to 120 minutes, more preferably at 600 to 950° C. for 30 to 90 minutesand particularly preferably at 700 to 900° C. for 30 to 60 minutes. Whenheating temperature is too low or heating time is too short, theobtained calcinated wet silica may cause insufficient improvementeffects of heat resistance and tensile stress of the obtainedcross-linked rubber. When heating temperature is too high or heatingtime is too long, the silica may be sintered to coarsen. Loss on heat ofthe calcinated wet silica is preferably 2 wt % or less. When loss onheat is too large, compression set of the obtained cross-linked rubbermay be increased, or heat resistance may be reduced.

The calcinated wet silica has surface silanol group concentration ofpreferably 3 groups/nm² or less, more preferably 2.5 groups/nm² or less.When the surface silanol group concentration is too large, tensilestress and heat resistance of the obtained cross-linked rubber areliable to be reduced. Since the calcination of wet silica reducessurface silanol group concentration, it results in improving affinity tothe nitrile rubber (a). Note that the surface silanol groupconcentration can be obtained by drying silica, followed by immersing indeuterated water, and determining free-proton amount in deuterated waterby NMR.

BET specific surface area of the calcinated wet silica is preferably 10to 500 m²/g, more preferably 30 to 300 m²/g and particularly preferably50 to 200 m²/g. When BET specific surface area of the calcinated wetsilica is too large, viscosity of the cross-linkable nitrile rubbercomposition is liable to increase to cause reduction in moldability.When BET specific surface area is too small, the obtained cross-linkedrubber may be reduced in tensile stress and heat resistance.

pH of the calcinated wet silica is preferably 3.5 to 8. When pH is toolow, crosslinking rate may be reduced and crosslink density of theobtained cross-linked rubber may be insufficient, resulting in largecompression set or reduced heat resistance. In contrast, too high pH maycause scorch at molding process.

A content of the silane-treated fumed silica and/or calcinated wetsilica (c) in the nitrile rubber composition of the present invention ispreferably 5 to 300 parts by weight, more preferably 10 to 200 parts byweight and particularly preferably 20 to 100 parts by weight per 100parts by weight of the nitrile rubber (a). When the content of thecomponent (c) in the nitrile rubber composition is too small, theobtained cross-linked rubber is liable to have insufficient improvementeffects of tensile stress and heat resistance, and large compressionset. In contrast, too large content of the component (c) in the nitrilerubber composition may cause to reduce workability of the nitrile rubbercomposition.

Note that weight ratio of the α,β-ethylenically unsaturated carboxylicacid metallic salt (b) and silica (c) in the nitrile rubber compositionof the present invention is, in terms of a value of (b)/(c), preferably1/100 to 200/100, more preferably 10/100 to 100/100.

Optional Components

The nitrile rubber composition of the present invention may include, ifneeded, other components in addition to the above component (a),component (b) and component (c). As such other components, although notparticularly limited, for example, polyether and organic metallic salthaving a fluoro group and/or sulfonyl group, etc., may be mentioned.Particularly, by using these polyether and organic metallic salt havinga fluoro group and/or sulfonyl group in combination, the obtainedcross-linked rubber may be improved in antistatic characteristic.

As the polyether usable in the present invention, any polymer havingplurality of ether bonds in the main chain can be used withoutparticular limitation, and a preferred example thereof is a (co)polymercontaining monomer unit(s) of the ethylene oxide and/or propylene oxidein a total amount of preferably 10 mol % or more, more preferably 40 mol% or more, further preferably 60 mol % or more and particularlypreferably 80 mol % or more. When the total amount of the monomerunit(s) of the ethylene oxide and/or propylene oxide is too small,antistatic effect of the obtained cross-linked rubber is liable to bereduced.

Note that when including the polyether, it is preferable to use thosehaving both ethylene oxide monomer unit and propylene oxide monomerunit, to thereby further improve the effect attained by additionthereof.

The polyether containing an ethylene oxide monomer unit has an effect tostabilize a metallic ion because of coordination of an ionized metal inthe ethylene oxide monomer unit. Then, the resultant segmental movementof the ethylene oxide monomer unit has such function to transport theion. However, too many ethylene oxide monomer units may result incrystallization to cause to inhibit segmental movement of the molecularchain. Therefore, by using the polyether further containing propyleneoxide monomer units as well, the above crystallization can be prevented,and antistatic effect can be improved.

Also, if the polyether contains a carbon-carbon unsaturated bond, it canbe cross-linked with the nitrile rubber (a), resulting in toughening theobtained cross-linked rubber, which is preferable. To obtain thepolyether containing a carbon-carbon unsaturated bond, it is preferableto copolymerize monomers containing a carbon-carbon unsaturated bond(which is hereinafter referred to as “unsaturated bond containingmonomer”), and it is particularly preferable to copolymerize allylglycidyl ethers because of its high copolymerizability.

As the polyether, because of further significant antistatic effect, aratio of the ethylene oxide monomer unit, propylene oxide monomer unitand unsaturated bond containing monomer unit “ethylene oxide monomerunit/propylene oxide monomer unit/unsaturated bond containing monomerunit” is preferably 50 to 99 mol %/0.5 to 49.5 mol %/0.5 to 20 mol %,more preferably 60 to 96 mol %/2 to 38 mol %/2 to 10 mol %. Note thatthe polyether can be obtained, for example, by a conventionally-knownpolymerization method such as solution polymerization by using aconventionally-known polymerization catalyst as a ring-openingpolymerization catalyst for an oxirane compound.

A weight-average molecular weight of the polyether is preferably 10,000or more, more preferably 10,000 to 1,000,000 and particularly preferably50,000 to 800,000. When the weight-average molecular weight is toosmall, bleeding may occur in the obtained cross-linked rubber, and whenthe weight-average molecular weight is too large, workability of thenitrile rubber composition may be reduced.

A content of the polyether in the nitrite rubber composition of thepresent invention is preferably 0.1 to 20 parts by weight, morepreferably 0.5 to 10 parts by weight and particularly preferably 1 to 10parts by weight per 100 parts by weight of the nitrile rubber (a). Whenthe content of the polyether is too small, antistatic effect of theobtained cross-linked rubber is liable to be decreased. In contrast, toolarge content may cause bleeding.

As the organic metallic salt having a fluoro group and/or sulfonylgroup, an organic alkali metallic salt and organic alkaline-earthmetallic salt having a fluoro group and/or sulfonyl group arepreferable, the organic alkali metallic salt is more preferable, and anorganic lithium salt is particularly preferable.

As an example of the above organic lithium salt, LiCF₃SO₃, LiN(SO₂CF₃)₂[compound name: lithium-bis(trifluoromethanesulfonyl)imide],LiC(SO₂CF₃)₂, LiCH(SO₂CF₃)₂, LiSF₅CF₂SO₃, Li[{OCH(CF₃)₂}Nb], etc., maybe mentioned.

As mentioned above, when the polyether contains an ethylene oxidemonomer unit, the ethylene oxide monomer unit stabilizes a metallic ionand also causes segmental movement to have function to transport theion. Therefore, when using the polyether and the organic metallic salthaving a fluoro group and/or sulfonyl group, it is preferable to applyprocedure including preliminarily mixing these sufficiently and thenmixing the other components constituting the nitrile rubber composition.Note that a product “Sankonol TBX-8310” manufactured by Sanko ChemicalInd. Co., Ltd. is commercially available as a mixture of ethyleneoxide-propylene oxide-allyl glycidyl ether copolymer and an organiclithium salt having a fluoro group and/or sulfonyl group, so that it issuitable to use in the above procedure.

A content of the organic metallic salt having a fluoro group and/orsulfonyl group in the nitrile rubber composition of the presentinvention is preferably 0.01 to 10 parts by weight, more preferably 0.2to 8 parts by weight and particularly preferably 0.5 to 6 parts byweight per 100 parts by weight of the nitrile rubber (a). When thecontent of the organic metallic salt having a fluoro group and/orsulfonyl group in the nitrile rubber composition of the presentinvention is too small, antistatic effect of the obtained cross-linkedrubber is liable to be reduced. In contrast, too large content may causeto reduce workability of the nitrile rubber composition.

Cross-Linkable Nitrile Rubber Composition

The cross-linkable nitrile rubber composition of the present inventioncomprises the above nitrile rubber composition and a crosslinking agent(d).

As the crosslinking agent (d), there may be used anyconventionally-known crosslinking agent, normally used for crosslinkingin a rubber, such as organic peroxide, polyamine compound, polyvalentepoxy compound, polyvalent isocyanate compound, aziridine compound,sulfur compound, basic metallic oxide and organometallic halide. Amongthese, organic peroxide and polyamine compound are preferable, andorganic peroxide is more preferable since it can easily give across-linked rubber with large elongation.

As the organic peroxide, normally, any of those used as a vulcanizingagent in rubber processing can be used without limitation. As an exampleof the organic peroxide, dialkyl peroxides, diacyl peroxides,peroxyesters, etc., may be mentioned.

As the dialkyl peroxides, dicumyl peroxide, di-t-butyl peroxide,1,3-bis(t-butylperoxyisopropyl)benzene, etc., may be mentioned.

As the diacyl peroxides, dibenzoyl peroxide, diisobutyryl peroxide,etc., may be mentioned.

As the peroxyesters, t-butyl peroxypivalate, t-butyl peroxyisopropylpercarbonate, etc., may be mentioned.

A content of the crosslinking agent (d) in the cross-linkable nitrilerubber composition of the present invention is preferably 0.2 to 20parts by weight, more preferably 1 to 15 parts by weight andparticularly preferably 1.5 to 10 parts by weight per 100 parts byweight of the nitrile rubber (a). When the content of the crosslinkingagent (d) is too small, it may be impossible to obtain a cross-linkedrubber with excellent mechanical properties and small compression set.In contrast, too large content may cause to reduce resistance to fatigueof the cross-linked rubber.

The cross-linkable nitrile rubber composition of the present inventioncan include a compounding agent normally used in the rubber processingfield in addition to the above nitrile rubber (a), α,β-ethylenicallyunsaturated carboxylic acid metallic salt (b), silane-treated fumedsilica and/or calcinated wet silica (c), crosslinking agent (d), and theoptionally-added polyether and organic metallic salt having a fluorogroup and/or sulfonyl group. As the compounding agent, for example,there may be mentioned a reinforcement filler such as carbon black,anon-reinforcement filler such as calcium carbonate and clay, a silanecoupling agent, an anti-aging agent, a light stabilizer, anantiscorching agent such as a primary amine, a plasticizer, a processingaid, a lubricant, an adhesive, a lubricating agent, a flame-retardant, afungicide, an acid acceptor, an antistatic agent, a coloring agent, acrosslinking accelerator, cross-linking auxiliaries, a cross-linkingretardant, etc. Amounts of these compounding agents are not particularlylimited as far as they are in the range not to disturb the purpose andeffects of the present invention, and suitable amounts for the purposeof blending can be appropriately selected.

Also, the cross-linkable nitrile rubber composition of the presentinvention may include rubbers other than the nitrile rubber (a) withinthe range not to disturb the purpose and effects thereof. A content ofthe other rubbers is preferably 50 parts by weight or less, morepreferably 10 parts by weight or less per 100 parts by weight of thenitrile rubber (a).

Production Methods of Nitrile Rubber Composition and Cross-LinkableNitrile Rubber Composition

The nitrile rubber composition and cross-linkable nitrile rubbercomposition of the present invention are produced by mixing each of theabove components preferably in nonaqueous system. A method for mixing isnot limited, but in the case of the cross-linkable nitrile rubbercomposition, it is preferable that components excluding the crosslinkingagent, etc., which are unstable to heat, are primarily kneaded in amixer such as a Bambury mixer, intermixer and kneader, and thensecondarily kneaded after transferring it into a roll, etc., and addingthe crosslinking agent, etc.

The nitrile rubber composition of the present invention has Mooneyviscosity [ML₁₊₄ (100° C.)] (compound Mooney) of preferably 15 to 200,more preferably 20 to 150. Also, the cross-linkable nitrile rubbercomposition of the present invention has Mooney viscosity [ML₁₊₄ (100°C.)] (compound Mooney) of preferably 15 to 150, more preferably 30 to120. Since the cross-linkable nitrile rubber composition of the presentinvention has the above compound Mooney, it is excellent in moldingprocessability. Note that the above Mooney viscosity is measured inaccordance with JIS K6300.

Cross-Linked Rubber

The cross-linked rubber of the present invention is obtained bycrosslinking the above cross-linkable nitrile rubber composition.

To obtain the cross-linked rubber by crosslinking, the cross-linkablenitrile rubber composition is molded by using a molding machine suitablefor a desired rubber shape, such as an extruder, an injection moldingmachine, a compactor, a roll and the like, and fixing the shape as across-linked rubber by crosslinking reaction. It may be cross-linkedafter molding or simultaneously. Molding temperature is preferably 10 to200° C., more preferably 25 to 120° C. Crosslinking temperature ispreferably 100 to 200° C., more preferably 130 to 190° C., andcrosslinking time is preferably 1 minute to 24 hours, more preferably 2minutes to 1 hour.

Also, depending on the shape, size, etc., of the cross-linked rubber,the inside may not be sufficiently cross-linked even if the surface iscross-linked, so that it may be secondarily cross-linked by furtherheating.

The cross-linked rubber of the present invention is excellent inmechanical properties such as tensile stress and small in compressionset. Particularly, it shows low rate of change in elongation in heataging test, indicating further improvement in heat resistance.

Since it has the above properties, the cross-linked rubber of thepresent invention can be used in wide range of application subjected torepeated strong shear stress including a variety of belts such as powerdriving flat belt, conveyer belt, V belt, timing belt and toothed belt;a variety of seal materials such as valve and bubble sheet, BOP (BlowOutPreventer), platter, O-ring, packing, gasket, diaphragm and oil-seal;attenuation rubber components such as cushion material, dynamic damper,rubber coupling, air spring and vibration-proofing materials; a varietyof hoses such as fuel hose, oil hose, marine hose, riser and flow line;a variety of rolls such as printing roll, iron-making roll, papermakingroll, industrial roll and roll for business equipment; a variety ofboots such as CVJ boots and propeller shaft boots; etc., as well as dustcover, automobile interior member, jacketed cable and shoe sole, andparticularly, it is useful for a belt or seal material.

EXAMPLES

Hereinafter, the present invention will be specifically described basedon Examples and Comparative Examples, but the present invention is notlimited to these examples. Note that “parts” in the following indicatesa weight base unless otherwise particularly designated.

Tests and evaluations were performed as below.

(1) Iodine Value

Iodine value was measured in accordance with JIS K6235.

(2) Mooney Viscosity [ML₁₊₄ (100° C.)]

Mooney viscosity (polymer Mooney) of the nitrile rubber was measured inaccordance with JIS K6300.

(3) Normal Physical Properties (Tensile Strength and Elongation)

The crosslinkable nitrile rubber composition was placed in a mold with alength of 15 cm, a width of 15 cm and a depth of 0.2 cm, andcross-linked at 170° C. for 20 minutes at a pressed pressure of 10 MPa,to prepare a test specimen. Tensile strength and elongation of thecross-linked rubber were measured in accordance with JIS K6251 by usingthe obtained test specimen.

(4) Heat Aging Test

By using a test specimen produced by crosslink as in the above (3),tensile strength, elongation and rate of change in elongation (%) at150° C. after 24 hours were obtained in accordance with JIS K6257(normal oven method).

Example 1

100 parts of nitrile group containing highly-saturated copolymer rubber[product name “Zetpol 2010L” manufactured by Nippon Zeon Co., Ltd., ahydrogenated acrylonitrile-butadiene copolymer rubber havingacrylonitrile unit content of 36%, iodine value of 11 and Mooneyviscosity (ML₁₊₄, 100° C.) of 85], 15 parts of zinc dimethacrylate, 1.5parts of 4,4′-di-(α,α′-dimethylbenzil)diphenylamine (product name“Naugard 445”, Crompton-Uniroyal Chemical) as an anti-aging agent, 1.5parts of zinc salt of 2-mercaptobenzimidazole (NoCRAC MBZ manufacturedby Ouchi Shinko Chemical Industrial), 40 parts of silica (1) (see thefollowing Note *1), 10 parts of zinc oxide (zinc white No. 1) as acrosslinking accelerator, 8 parts of trimellitic acid ester (productname “ADEKA CIZER C8” manufactured by Asahi Denka Company Limited) as aplasticizer and 8 parts of 40% product of 1,3-bis(t-butylperoxyisopropyl)benzene (Vulcup 40KE by GEO Specialty Chemicals Inc., anorganic peroxide) (3.2 parts of pure organic peroxide) were kneaded at50° C. in a open roll to prepare a cross-linkable nitrile rubbercomposition. A cross-linked rubber obtained by this was subject to eachtest and evaluation of normal physical properties (tensile strength,elongation, 100% tensile stress) and heat aging test (tensile strength,elongation, rate of change in elongation). The results are shown inTable 1.

Note *1: The silica (1) was silane-treated fumed silica (product name“Aerosil R972” manufactured by Nippon Aerosil Co., Ltd., having specificsurface area of 110 m²/g and pH=3.8). This was obtained by treating“Aerosil 130” manufactured by Nippon Aerosil Co., Ltd. withdimethyldichlorosilane.

Example 2

Except for using silica (2) (see the following Note *2) instead of thesilica (1), a cross-linkable nitrile rubber composition was produced asin the Example 1. Same tests and evaluations were performed as in theExample 1, and the results are shown in Table 1.

Note *2: The silica (2) is calcinated wet silica (product name “CarplexCS-7”, manufactured by Shionogi & Co., Ltd., having specific surfacearea of 133 m²/g and pH=6.9).

Comparative Example 1

Except for using silica (3) (see the following Note *3) instead of thesilica (1), a cross-linkable nitrile rubber composition was produced asin the Example 1. Same tests and evaluations were performed as in theExample 1, and the results are shown in Table 1.

Note *3: The silica (3) is wet silica obtained by a precipitation method(product name “Nipsil VN-3” manufactured by Tosoh Silica Corporation,having specific surface area of 195 m²/g).

Comparative Example 2

Except for adding 2 parts of vinyl tris(β-methoxyethoxy)silane (productname “A-172” manufactured by Nippon Unicar Company Limited) as a silanecoupling agent, a cross-linkable nitrile rubber composition was producedas in the Comparative Example 1. Same tests and evaluation wereperformed as in the Example 1, and the results are shown in Table 1.

Comparative Example 3

Except for using silica (4) (see the following Note *4) instead of thesilica (1), a cross-linkable nitrile rubber composition was produced asin the Example 1. Same tests and evaluations were performed as in theExample 1, and the results are shown in Table 1.

Note *4: The silica (4) is fumed silica (product name “Aerosil 200”manufactured by Nippon Aerosil Co., Ltd., having specific surface areaof 200 m²/g).

Comparative Example 4

Except for not adding 15 parts of zinc dimethacrylate, a cross-linkablenitrile rubber composition was produced as in the Example 1. Same testsand evaluations were performed as in the Example 1, and the results areshown in Table 1.

TABLE 1 Examples Comparative Examples 1 2 1 2 3 4 Components Zincdimethacrylate 15 15 15 15 15 — (parts) Silica (1) “Aerosil R972” 40 — —— — 40 Silica (2) “Carplex CS-7” — 40 — — — — Silica (3) “Nipsil VN-3” —— 40 40 — — Silica (4) “Aerosil 200” — — — — 40 — Silane Coupling Agent“A-172” — — — 2 — — Normal Tensile Strength (MPa) 26.5 25.5 25.7 27.025.3 24.1 Physical Elongation (%) 470 430 480 460 460 430 Properties100% Tensile Stress (MPa) 7.3 8.5 6.8 6.8 7.3 3.2 Heat Aging TensileStrength (MPa) 27.6 27.6 28.0 27.2 26.6 25.0 Test Elongation (%) 390 350270 260 290 360 Rate of Change in Elongation (%) −17 −19 −44 −43 −37 −16

As shown in Table 1, the cross-linked rubber obtained by crosslinkingthe nitrile rubber composition satisfying the requirements of thepresent invention had sufficient normal physical properties (tensilestrength, elongation and 100% tensile stress), which were hardlydeteriorated even by heating at 150° for 24 hours, indicating good heatresistance (Examples 1 and 2).

On the other hand, when using wet silica or the same added with a silanecoupling agent, the resulting cross-linked rubbers were significantlyreduced in elongation due to heat, indicating inferior heat resistance(Comparative Examples 1 and 2).

Also similarly, using fumed silica without silane-treatment, theprovided cross-linked rubber was low in heat resistance (ComparativeExample 3).

Further, the cross-linked rubber obtained by using the nitrile rubbercomposition not including zinc dimethacrylate was significantly inferiorin tensile stress (Comparative Example 4).

The invention claimed is:
 1. A nitrile rubber composition comprising anitrile rubber (a) including an α,β-ethylenically unsaturated nitrilemonomer unit and having iodine value of 100 or less, anα,β-ethylenically unsaturated carboxylic acid metallic salt (b), andcalcinated wet silica (c).
 2. The nitrile rubber composition as setforth in claim 1, wherein a content of said α,β-ethylenicallyunsaturated carboxylic acid metallic salt (b) is 3 to 120 parts byweight per 100 parts by weight of said nitrile rubber (a).
 3. Thenitrile rubber composition as set forth in claim 1 or 2, wherein acontent of said calcinated wet silica (c) is 5 to 300 parts by weightper 100 parts by weight of said nitrile rubber (a).
 4. The nitrilerubber composition as set forth in claim 1, wherein saidα,β-ethylenically unsaturated carboxylic acid metallic salt (b) is azinc salt and/or magnesium salt of unsaturated monocarboxylic acid. 5.The nitrile rubber composition as set forth in claim 4, wherein saidα,β-ethylenically unsaturated carboxylic acid metallic salt (b) is azinc dimethacrylate.
 6. The nitrile rubber composition as set forth inclaim 1, wherein said calcinated wet silica is adjusted to have asurface silanol group concentration of 3 groups/nm² or less bycalcinating wet silica.
 7. A cross-linkable nitrile rubber compositioncomprising the nitrile rubber composition as set forth in claim 1 and acrosslinking agent (d).
 8. The cross-linkable nitrile rubber compositionas set forth in claim 7, wherein a content of said crosslinking agent(d) is 0.2 to 20 parts by weight per 100 parts by weight of the nitrilerubber (a).
 9. The cross-linkable nitrile rubber composition as setforth in claim 7, wherein said crosslinking agent (d) is an organicperoxide.
 10. A cross-linked rubber obtained by crosslinking thecross-linkable nitrile rubber composition as set forth in claim
 7. 11.The cross-linked rubber as set forth in claim 10 which is a belt or sealmaterial.